A paper machine woven fabric band of this type is disclosed in WO 2006/020414 A1. It has two woven fabric layers disposed one over the other, the first woven fabric layer or the one on the paper side being formed by first transverse threads and first longitudinal threads, and the second woven fabric layer or the one on the machine side being formed by second transverse and second longitudinal threads. The first longitudinal threads bind a transverse thread of the second woven fabric layer within a repeat and in this way join the two woven fabric layers. The second longitudinal threads only bind within the lower woven fabric layer by floating on the lower side over a plurality of second transverse threads and at the binding points binding just one second transverse thread respectively on the inside. The floats form the outer side of the second woven fabric layer and in this way improve the abrasion resistance. Here some of the longitudinal threads should be made of polyamide or polyester.
A disadvantage with this paper machine woven fabric band is that the second longitudinal threads must absorb a large part of the tensile forces acting upon the woven fabric band in the paper machine and that the cross-section of these second longitudinal threads and so the tensile strength of the latter decreases due to abrasion as the operating time increases.
In GB 1 562 284 a single-layer paper machine woven fabric band is disclosed wherein, as regards their material, the longitudinal threads comprise two groups, namely a first longitudinal thread group which first and foremost should guarantee the dimensional stability of the band and for which polyester is proposed as a material, and a second longitudinal thread group which is characterized by greater abrasion resistance in relation to the longitudinal threads of the first group, and are therefore produced, for example, from polyamide. It is proposed here to allow the longitudinal threads to extend in two layers, the polyester threads in the layer on the paper side and the polyamide threads in the layer on the machine side.
In EP 1 025 306 B1 a single-layer paper machine woven fabric band is disclosed the structure of which comprises interconnected longitudinal and transverse threads, longitudinal threads additionally being woven in as reinforcement threads with floats extending on the lower side of the band, said threads improving abrasion resistance and reducing friction with respect to the machine parts and so the expenditure of energy when circulating in the paper machine. These longitudinal threads can be made, for example, of polyamide, while the other longitudinal threads and also the transverse threads are made of polyester or polyethylene naphthalate.
Single-layer paper machine woven fabric bands offer the advantage that they are lower in weight and are less stiff than two- or three-layer woven fabric bands, and so are easier to install in the paper machine (see EP 1 025 306 B1, section [0020]), but have the disadvantage that the paper side is not formed independently of the machine side and so an optimal woven fabric structure can not be achieved.
In DE 10 2004 016 640 B3 a paper machine woven fabric band is disclosed, which has two woven fabric layers disposed one over the other. The first woven fabric layer destinated to be the paper side layer is formed by first transverse threads and first longitudinal threads, and the second woven fabric layer destinated to be the machine side layer is formed by second transverse and second longitudinal threads. Both woven fabric layers are joined by separate and additional binder yarns extending transversely and binding a longitudinal thread of the first woven fabric layer and then a second longitudinal thread of the second woven fabric layer one after another. In the description it is suggested that binder yarns can also be provided as machine direction yarns without disclosing, how they are bind with first and second transverse threads of the two woven fabric layers.
The disadvantage of this paper machine band is the same as of the band disclosed in WO 2006/020414 A1. The second longitudinal threads must absorb large part of the tensile forces acting upon the woven fabric band, and these second longitudinal threads are subject to abrasion over their lifetime, due to floating segments projection on the machine side of the band.
The object which forms the basis of the invention is to design a woven fabric band, in particular for use in a paper machine, such that the greatest possible freedom is obtained when forming the outer sides and so flexibility in adaptation to the respective application is obtained, and at the same time a high degree of tensile strength and high abrasion resistance is achieved here.
In order to achieve this object one starts according to the invention with a woven fabric band which is formed from two woven fabric layers, and with which, consequently, the freedom of form on both outer sides—with a paper machine woven fabric band the paper side on the one hand and the machine side on the other hand—is very great in comparison to single-layer woven fabric bands, and so both outer sides can be optimally adapted to the respective requirements. In order at the same time to be able to achieve a high level of rigidity and high abrasion resistance here, according to the invention the second longitudinal threads form a first longitudinal thread group with outer second longitudinal threads and a second longitudinal thread group with inner second longitudinal threads, the outer second longitudinal threads defining the outer side of the second woven fabric layer and the inner second longitudinal threads extending totally within the woven fabric band. Therefore, according to the invention (at least) three longitudinal thread groups are formed, the first and the second longitudinal thread group with the second transverse threads forming the second woven fabric layer, and the third longitudinal thread group serving to form the first woven fabric layer. Therefore, all three longitudinal thread groups can be adapted optimally to their respective function, both with regard to their binding into the woven fabric band and the woven fabric layers assigned to them and with regard to their material.